Nickel chromium alloys having high creep strength at high temperatures



y 1955 H. E. GRESHAM ET AL 2,712,498

NICKEL CHROMIUM ALLOYS HAVING HIGH CREEP STRENGTH AT HIGH TEMPERATURES Filed May 25, 1949 E n5 0 g- L) u.

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2,712,498 NICKEL CHROMIUM ALLOYS HAVING HIGH CREE? STRENGTH AT HIGH TEMPERATURES Harold Ernest Gresham, Little Eaton, Adam Dunlop, Alvaston, and Marcus Alan Wheeler, Darley Abbey, England, assignors to Rolls-Royce Limited, Derby, England, a British company Application May 23, 1949, Serial No. 94,816 Claims priority, application Great Britain June 1, 1948 8 Claims. (Cl. 75-171) Our invention relates to alloys from which to make engineering parts capable of withstanding great stress at high temperatures, ranging for instance between 700 and 1050 C. As examples of such engineering parts we may mention the blades and other parts of internal combustion gas turbines. Claim is hereby made for the benefit of the filing date of patent application numbered 14,823 of 1948 filed in Great Britain lune 1, 194-8.

In our research work on alloys of this type we started from the nickel-chromium base alloys which are commonly used in the productions of high duty engine parts and which often contain cobalt and as hardening constituents aluminum and titanium. Like other workers in this field, we have had the experience that sometimes quite trifling differences in the constitution of this type of alloys-diiferences such as will normally spring up even in careful foundry practice-lead to alloys of widely divergent properties so that it appeared to be impossible to predicate with certainty the quality of an engineering part made out of an alloy of this type, until it had been actualproduction of high duty engineering parts of the kind here in view from a stage of comparative uncertainty to the level of an altogether controllable production leading to predetermined properties of the products.

We have found that there exists a definite correlation between the properties of a nickel-chromium-cobalt alloy, in which the cobalt is contained within the range of about 10 and per cent by weight of the complete alloy, and chromium within the range of 10 and 30 per cent and which contains, besides aluminum and titanium, also molybdenum within the ranges of 2 to 18 per cent molybdenum, 0.2 to 8.6 per cent aluminum and 0.2 to 4.4 per cent titanium-and the total percentage of molybdenum, aluminum and titanium present.

We have found that this type of alloys with fully predictable properties will be produced, if the percentages of the three alloying metals Mo-Al Ti are so chosen that by adding once the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium, a figure is reached which ranges between 16 and 20.

The range of 16-20 arrived at according to the formula 1 percent Mo+2 Al+4 percent Ti we will call for convenience the balance factor.

One of the principal advantages gained by the use of a composition balanced in accordance with our discovery is the safe control of the creep stren For it is known that a number of well known alloys, in current use where high creep strength is required, suffer from considerable variation in their properties, a circumstance which we found to be due to the use of unbalanced compositions.

The great advantages resulting in the use of our balrates a't 2,712,493 i atented July 5, 1955 ance factor formula are illustrated by the addition of molybdenum, aluminum and titanium in the following examples, in each of which the alloy contains 20% chromium and 20% cobalt, the balance being nickel. These alloys were tested at 815 C. under a load of 15 long tons/sq. in.

it will be noted that, when the balance factor is below 16, as is the case in examples g, and h, the period of time which elapsed before fracture, is short. On the other hand examples a, b, c, a and e, clearly indicate the great advantage gained by the use of our formula in connection with alloys of widely different compositions.

The use of our formula is equally of great value in controlling the properties of alloys held to a much narrower range of composition. In making a considerable number of casts from alloys of the type containing, besides 20% Cr and 20% Co, 6 per cent Mo, 0.9 per cent Al and 2.4 per cent Ti, the balance being Ni, in varying the percentage of Mo by 10.4 per cent, of Al by i031 per cent and of Ti ;0.3 per cent, we found that, if all errors were towards the minus side and the balance factor dropped to 15.2, the creep strength was poor. On the other hand, Whenever one of the three elements kept on the higher level, the creep properties were greatly improved.

A series of twelve nickel alloys were prepared having the approximate composition 20% chromium and 20% cobalt, while the percentage of Mo ranged from 5.5 to 6.5, that of Al from 0.4 to 1.2, that of Ti from 2.2 to 2.7. Such ranges of composition would be expected to yield alloys having very much the same properties. However the very sensitive creep test brought to light great diifcrences in the behaviour of different alloys and as plotted in the form of twelve points marked x on the chart hereto annexed, the life to fracture (hours) of said alloys under the abovementioned test conditions (15 long tons/sq. in. at 815 C.) ranged from a low of about 14 to a high of about 190. Such variations in the creep results are well known to metallurgists. They constitute a great problem in the bulk production of creep-resisting alloys. However, as soon as these small variations of composition are examined in reference to our formula, their real significance becomes obvious and the need for a balanced composition becomes apparent. On the accompanying chart the above-mentioned twelve alloys have been plotted according to their respective balance factors, which varied from about 15 to about 19.6. When thus plotted it is at once seen that a relationship exists between a balance factor ranging from 16 to 20 and the attainment of high creep strength. The line 10 which is drawn on the chart following generally the rising trend of the several points marked x does not represent the results of any tests other than those indicated by the points themselves; it is intended only to emphasize and point out the abovementioned relationship which we have discovered. By using the formula, one is in the position to choose a composition which, while allowing for normal variation during manufacture, results in consistently high creep properties being obtained.

If the balance factor exceeds 20, hot working of the alloys becomes difficult. Therefore the use of our for- As to the presence of other metals, the nickel may be replaced up to 10 per cent by vanadium, and there may further be present:

Per cent Niobium -4 Manganese 02 Silicon 0-1 Iron 0-5 Carbon 0-0.5 Nitrogen O0.25

While niobium may be beneficial, since it improves creep strength, and manganese and silicon are added mainly as cleansing elements, iron, which occurs as an impurity in many commercial metals, should be kept as low as possible. Carbon, also present as an impurity, should also remain within the limits indicated above.

For cleansing and de-oxidation purposes up to 0.5, in all, of the elements calcium, magnesium, cerium and other rare earth metals or misch-metal may be present.

The alloys should be heat-treated to increase hardness and strength and assist in fabrication. A suitable heattreatment includes preliminary heating at 1050 to 1250" C. during 2 to 20 hours, and this should be followed by reheating at 7001000 C. for 2 to 50 hours to produce the highest degree of hardening.

The wide control of hardness by a combination of balanced composition and heat treatment is an important feature of our invention.

For maximum creep strength it is essential to compound the alloy according to the formulas given above so as to ensure that the balance factor remains within the range of 16-20 and that the molybdenum, aluminum and titanium are all present simultaneously.

We wish to point out that various changes may be made in certain substances, steps and figures hereabove mentioned without departing from our invention or sacrificing the advantages thereof.

We claim:

1. A hot-workable nickel-chromium base alloy having a life to fracture under load of 15 long tons per square inch at 815 C. substantially in excess of 38 hours, said alloy consisting by weight approximately of 10% to 40% cobalt, 10% to 30% chromium, each of the three hardening elements molybdenum, aluminum and titanium Within the respective approximate ranges 2% to 18% molybdenum, 0.2% to 8.6% aluminum and 0.2% to 4.4% titanium, the impurities iron and carbon not exceeding about iron and 0.5% carbon, and the balance essentially nickel, said alloy being characterized in that the sum of the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium equals a figure within the range of 16 to 20.

2. A hot-workable nickle-chromium base alloy having a life to fracture under load of 15 long tons per square inch at 815 C. substantially in excess of 38 hours, said alloy consisting by weight approximately of to 40% cobalt, 10% to 30% chromium, each of the three hardening elements molybdenum, aluminum and titanium within the respective approximate ranges 2% to 18% molybdenum, 0.2% to 8.6% aluminum and 0.2% to 4.4% titanium, the impurities iron and carbon not exceeding about 5% iron and 0.5% carbon, up to 10% vanadium, and the balance essentially nickel, said alloy being characterized in that the sum of the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium equals a figure within the range of 16 to 20.

3. A hot-workable nickel-chromium base alloy having a life to fracture under load of long tons per square inch at 815 C. substantially in excess of 38 hours, said alloy consisting by weight approximately of cobalt, 20% chromium, each of the three hardening elements molybdenum, aluminum and titanium within the respective approximate ranges 5.5% to 6.5% molybdenum, 0.4% to 1.2% aluminum and 2.2% to 2.7% titanium,

the impurities iron and carbon not exceeding about 5% iron and 0.5% carbon, and the balance essentially nickel, said alloy being characterized in that the sum of the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium equals a figure within the range of 16 to 20.

4. A hot-workable nickel-chromium base alloy having a life to fracture under load of 15 long tons per square inch at 815 C. substantially in excess of 38 hours, said alloy consisting by weight approximately of 10% to 40% cobalt, 10% to 30% chromium, 15.5% molybdenum,

0.9% aluminum, 0.2% titanium, and the balance essentially nickel, said alloy being characterized in that the sum of the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium equals a figure within the range of 16 to 20. 5. A hot-workable nickel-chromium base alloy having a life to fracture under load of 15 long tons per square inch at 815 C. substantially in excess of 38 hours, said alloy consisting by Weight aproximately of 10% to 40% cobalt, 10% to 30% chromium, 5.94% molybdenum, 3.75% aluminum, 0.9% titanium, and the balance essentially nickeL'said alloy being characterized in that the sum of the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium equals a figure within the range of 16 to 20. 6. A hot-workable nickel-chromium base alloy having a life to fracture under load of 15 long tons per square inch at 815 C. substantially in excess of 38 hours, said alloy consisting by weight approximately of 10% to 40% cobalt, 10% to 30% chromium, 6.10% molybdenum, 0.96% aluminum, 2.76% titanium, and the balance essentially nickel, said alloy being characterized in that the sum of the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium equals a figure within the range of 16 to 20. 7. A hot-workable nickel-chromium base alloy having a life to fracture under load of 15 long tons per square inch at 815 C. substantially in excess of 38 hours, said alloy consisting by weight approximately of 10% to 40% cobalt, 10% to 30% chromium, 6.12% molybdenum, 0.49% aluminum, 2.58% titanium, and the balance essentially nickel, said alloy being characterized in that the sum of the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium equals a figure within the range of 16 to 20.

8. A hot-workable nickel-chromium base alloy having a life to fracture under load of 15 long tons per square inch at 815 C. substantially in excess of 38 hours, said alloy consisting by weight approximately of 10% to 40% cobalt, 10% to 30% chromium, 3.50% molybdenum, 1.22% aluminum, 2.58% titanium, and the balance essentially nickel, said alloy being characterized in' that the sum of the percentage of molybdenum plus twice the percentage of aluminum plus four times the percentage of titanium equals a figure within the range of 16 to 20.

References Cited in the file of this patent UNlTED STATES PATENTS 2,048,165 Pilling et a1 July 21, 1936 2,103,500 Touceda Dec. 28, 1937 2,246,078 Rohn et al June 17, 1944 2,398,678 Thielemann Apr. 16, 1946 2,403,128 Scott et al. July 2, 1946 2,423,738 Thielemann July 8, 1947 2,570,194 Bieber et al. Oct. 9, 1951 FOREIGN PATENTS 583,845 Great Britain Jan. 1, 1947 607,616 Great Britain c- Sept. 2, 1948 OTHER REFERENCES Knight, Treatise in Materials and Methods, June 1946, pages 1557l563; 1562 relied upon. 

1. A HOT-WORKABLE NICKLE-CHROMIUM BASE ALLOY HAVING A LIFE TO FRACTURE UNDER LOAD OF 15LONG TONS PER SQUARE INCH AT 815* C. SUBSTANTIALLY IN EXCESS OF 38 HOURS, SAID ALLOY CONSISTING BY WEIGHT APPROXIMATELY OF 10% TO 40% COBALT, 10% TO 30% CHROMIUM, EACH OF THE THREE HARDENTHE ELEMENTS MOLYBDENUM, ALUMINUM AND TITANIUM WITHIN THE RESPECTIVE APPROXIMATE RANGES 2% TO 18% TO 4.4% DENUM, 0.2% TO 8.6% ALUMINUM AND 0.2% TO 4.4% TITANIUM, THE IMPURITIES IRON AND CARBON NOT EXCEEDING ABOUT 5% IRON AND 0.5% CARBON, AND THE BALANCE ESSENTIALLY NICKLE, SAID ALLOY BEING CHARACTERIZED IN THAT THE SUM OF THE PERCENTAGE OF MOLYBENUM PLUS TWICE THE PERCENTAGE OF ALUMINUM PLUS FOUR TIMES THE PERCENTAGE OF TITANIUM EQUALS A FIGURE WITHIN THE RANGE OF 16 TO
 20. 